Knowing who is phoning you before you answer their call is normal on mobile phones - but there's much more that could be done with the system when you're at home.

How one caller ID system can alert you where your call is coming from

If you call anyone from the pub payphone and try to make out that you're stuck in a meeting at work, there's a good bet that you could be rumbled.

An increasing number of people are subscribing to caller ID service, which provides the telephone number of the incoming call on their phone line.

While most people take for granted that all mobile phones display caller ID information when they ring, far fewer remember that when they call a home phone number the person who answers may already know who is calling, or that the call is coming from a payphone, not an office meeting room.

If you subscribe to caller display services offered by phone networks, the information sent down the telephone line before the first ring is actually quite rudimentary - the useful bits being the date, time, and telephone number of the person calling. It can also indicate that the call is from a payphone, is unavailable (perhaps because the call comes from overseas), or that the number has been withheld.

Limited usefulness

When Caller Display was launched in 1995 you had to have a special unit at home to receive it - either a small box plugged in to the telephone jack which would display the incoming caller's number, or a specially equipped phone. Unless you happened to be able to remember the phone number of everyone you knew, Caller Display's usefulness was severely limited.

It's simple to turn a home office computer into what a few years ago would have been an expensive call centre workstation

Now that's all changed. Connected to a home computer or television set top box, caller ID is a powerful tool for home businesses as well as control freaks who like to know who's calling before they answer the phone and those who don't want to be disturbed unnecessarily from their favourite television programme.

With a modem which is compatible with your particular phone operator's caller ID system and low cost software downloadable from the internet, it's simple to turn a home office computer into what a few years ago would have been an expensive call centre workstation.

Greeting customers

When anyone calls, the modem captures the caller ID information and sends it to the software, which looks up the number in its phone book and pops up the name and number of the caller and details of past dealings with that person on the computer screen. The computer can even announce the caller's name.

This enables you to greet important customers appropriately and have their details immediately to hand or, if you're particularly busy, to ignore non-urgent calls.

Landlines are feeling the influence of mobiles

Since the software knows exactly who is calling, it's possible to leave different answering machine messages for different people when you're out: friends receive a personal message, while business customers get a more formal one.

You can even leave a different message for one specific person - your drinking partner could receive a message to meet you in the pub, while any other callers are informed that you are in a meeting for the rest of the day, for example.

Caller ID information call also be used for call filtering, to avoid nuisance callers or anyone else you don't feel like talking to. Tell the software the relevant telephone number and your phone won't even ring when they call - instead they'll simply hear a pre-recorded message saying calls from that number are no longer accepted.

Withholding information

It is possible for anyone making a call to withhold the caller ID information that is normally sent down the phone line (on BT lines by dialling 141 before making the call) but caller ID software can also be made to reject all calls which withhold the caller ID information.

What if you don't have a computer at home but don't want to be disturbed by the mother-in-law (or father-in-law) when you've settled down in front of a film or a big game on the television?

Get a caller ID set top box which superimpose the name and number of incoming callers on your television screen - so you can decide before you answer whether you want to take the call (an elderly relative who may need help, for example) or an unwelcome chatterbox who'll make you miss the rest of the film.

BT alone estimates it has about 1.9 million customers for its caller display service. They may or may not be using the technology to its potential.

But when you phone the boss from the airport to say you're sick at the start of a long weekend, it's worth remembering that they just might be.

Add your views to this story using the form below.

The telling word in this article is "Subscribe". In 1995 my mobile phone displayed incomming telephone numbers from BT and Orange numbers. Now, at no extra cost, I get all networks and many international numbers at no extra cost yet we must still SUBSCRIBE to this service from BT.Chris Devine, UK

I think this is a really brilliant idea. I have caller display and find it useful. I would certainly use this service if it was not too expensive. Marilyn Lambert, United Kingdom

Caller ID is a good idea in principle, but the ability to voluntarily withold your number defeats much of the object - any caller up to no good will do this automatically. And while the option to reject calls from witheld numbers stops this problem, it also blocks genuine calls - e.g. I can't call home to tell my partner I'll be late, since my phone is routed through a private exchange and no ID is generated. Remove the option to withold, and this would be a genuinely powerful facility. Nigel, UK

I'm at univeristy in Scotland and depend on my mobile phone caller display to ensure that I don't answer my Mother's calls when I'm in the pub. That software would be a god-send, just imagine, next time I'm out for a session - "I'm sorry I can't answer your call Mother, I'm in the library!". So simple yet so effective!Matthew Maguire, Scotland

If you decide after the fact that you don't wish for your number to be available to someone through 1471, you can remove it by dialling 1475. Unfortunately you have to do this from the phone of the person you just called, which reduces its usefulness. I believe this was added in order to comply with the Data Protection Act. David, UK

Caller-ID is almost there but..
(1) not all UK networks transfer it across
(2) Company networks do not normally send out the required DDI number
(3) why pay for someting that mobiles give away free, as you also have to pay for the hardware. But I would still think about using it Nigel T, UK

Removing the option to withhold a phone number would affect people with ex-directory phone numbers - many of whom are ex-directory because of fears of violence from an ex-partner or because they have received malicious, obscene or threatening phone calls. Carol, UK

BT don't tell you that Caller ID does not work with BT broadband if you are on line, this is because of the action of the ADSL filters. So don't go paying for something that you won't be able to use all the time. Barry Henderson, UK

I think this isn't exactly before time. Since I started using my mobile to receive and make just about every call, I've got into the habit of declining a call if anyone calls me anonymously. Graeme Phillips, UK

A more mundane complaint is that I haven't got used to answering the phone in a new way. For example, if you can see on the caller display that your mate's calling you from his mobile phone, it feels absurd to answer the phone with a 'Hello?' - but it seems that whenever I get over this obstacle and answer the phone with a (say) 'Hi Nick, how are you?' it turns out to be someone else using his phone. So I generally stick with 'hello?' and pretend that I haven't checked who's calling before answering the phone. John Cookson, UK

Caller ID is the root of a big problem in Japan, nicknamed "wangiri". There are businesses that aim to catch you out by calling you and then hanging up. Curious to know who it was you call back and find you have dialled an adult chat line or some other premium rate number. Obviously you should avoid calling numbers you don't recognise but that's easy to say. Daniel, UK

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